I got my hands on an EI plasma cannon during an EI500 run (in which I loved the Makayev launcher and NAMI heavy launcher so much that I never used a howitzer), so I installed it and used it in Vori and Dantalion before I replaced it with a NAMI heavy to clear out IC 767. I used the plasma again in Heretic, where it did well against the Xenophobe fleet, but I swapped it out for the Qianlong to defeat the Iocrym Command ship. In the run in which I used the plasma cannon, I found just two of the EI plasma cannons in Ares shipyards and nowhere else (I use SotP only).

I'll describe the cannon as well as I can:
The plasma cannon feels like a recoiless cannon as it fires its plasma in a tightly concentrated blob, though it's still a cloud. It has a range of about 60 ls and has WMD 4 (on the 1-7 scale). It fires 3.8 shots per second.
It's level 10 and consumes 300MW, same as the Iocrym Fracture cannon and the Ares plasma archcannon.

The Fracture cannon is a fast, short ranged and a good station buster. The APA is a slow, very long ranged and a good station buster. But compared with these two weapons, Earth Industries' plasma cannon is average:

It has a longer range than the Fracture, but not that much;

Its firing rate and WMD rating are between the Fracture and APA. It appears to deal less damage per second than either;

It only takes one slot, same as the Fracture

It isn't tagged with NotForSale, but the system level scaling will mean that Adventure Outfitters will not stock any plasma cannons, so it seems only Ringer shipyards and Teraton trading posts have any chance of selling them in SotP. You might never find a plasma cannon made by EI, but you always have a chance of getting the Fracture or the APA if you can kill their respective ships.

To increase its strength I suggest increasing the WMD of the EI plasma cannon, so that it's even better against capital ships and stations. Maybe increase its range a little more to ~80ls and/or increase its damage to to the point where it can kill a Sandstorm gunship with one good shot.